Lung carcinomas, malignant melanomas, gliomas, neuroblastomas, pheochromocytomas, colon, renal, prostate and breast carcinomas and the like are aggressive forms of cancer, the early detection and treatment of which are of paramount importance. If left undetected or untreated, for several years or even months, the median survival time of patients having these types of cancers is dramatically reduced.
Of these cancers, lung cancer has led to the highest number of fatalities. In 1992 alone, lung cancer caused about 165,000 deaths within the United States. Two major types of lung carcinomas are responsible for most of these deaths: small cell lung carcinomas (SCLC) and nonsmall cell lung carcinoma (NSCLC).
SCLC is a neuroendocrine tumor that secretes several peptide growth factors including bombesin/gastrin releasing peptide (BN/GRP). SCLC is responsive to chemotherapy and radiation therapy, but relapse occurs frequently, and the median survival time is only about one year.
NSCLC accounts for about 75% of all lung cancer cases and encompasses a variety of carcinomas including adenocarcinomas, large cell carcinomas and squamous cell carcinomas. NSCLC tumors secrete transforming growth factor-alpha (TGF-.alpha.) to stimulate cancer cell proliferation. NSCLC is generally treated with chemotherapy and surgical resection. However, the median survival time for patients with NSCLC is only about 5 years.
Melanomas are among the most serious manifestations of skin cancer and lead to a greater number of fatalities than any other form of skin cancer. Melanomas can metastasize through the lymphatic system to regional nodes and then via the blood to secondary sites on the skin or in the liver, lungs and brain. Whereas the prognosis for superficial spreading melanomas can be quite good, there is a much poorer prognosis for nodular melanomas in which distant metastases frequently form.
Breast cancer is a major cause of death for women, and estrogen receptors have been reported to play a major role in the development and growth of breast tumors. Deprivation of estrogen is one of the clinically effective methods for the treatment of breast cancer patients. Several growth factors such as insulin-like growth factor I (IGF-1), transforming growth factors (TGF-.alpha. and -.beta.), epidermal growth factor (EGF), and platelet-derived growth factors have been shown to be involved in the growth and progression of human breast cancer cells. Some growth factors such as TGF-.beta. act as inhibitors of tumor growth. Despite the development of numerous antiestrogen compounds and other drugs, the clinical utility of antiestrogen is limited due to resistance by the tumor cells.
Many lives could be saved if lung carcinomas, melanomas, gliomas, neuroblastomas, pheochromocytomas, colon, prostate and renal carcinomas, breast tumors and the like were detected and treated at an early stage. Moreover, many patients are reluctant to undergo radical surgical or broad spectrum chemotherapy procedures which are frequently used to treat such cancers because these procedures can cause disfiguration and/or disablement.
Current techniques diagnose breast cancer by first identifying suspect tumors by single plane or 2D mammography screening. A biopsy is then required to differentiate tumors from other lesions. In the United States alone, 21 million mammographies are performed each year; 700,000 suspect tumors are biopsied and 182,000 women are diagnosed with breast cancer. This suggests that 400,000-500,000 women are subject to unnecessary biopsy each year.
Accordingly, an outstanding need exists for highly selective and non-invasive procedures permitting early detection and treatment of cancer.
A variety of radiopharmaceuticals have been evaluated for diagnostic imaging. For example, Michelot, J. M. et al. (1991 J. Nucl. Med. 32:1573-1580; Meyniel G. et al. (1990 C.R. Acad. Sci. Paris 3II (I): I3-18; and French Patent Publication No. 2,642,972 by Morean et al. have disclosed .sup.123 I! and .sup.125 I!N-(diethylaminoethyl)4-iodobenzamide (i.e. IDAB) for imaging malignant melanoma in humans. Unfortunately, the synthesis of IDAB is problematic and, more significantly, IDAB is taken up in high concentrations by non-melanoma cells in the liver and lung. Accordingly, IDAB does not have optimal specificity for melanoma cells and its uptake by non-tumor cells undermines its utility for routine screening of cancer.
The present invention provides compounds and complexes which bind with high specificity and affinity to receptors on a cancer cell surface. One such receptor is a sigma receptor. Sigma receptors are known to be present on neural tissues and certain immortalized neuroblastoma and glioma cell lines (Walker et al., 1990 Pharmacol. Reviews 42: 355-400; and Villner et al., Multiple Sigma and PCP Receptor Ligands: Mechanisms for Neuromodulation and Neuroprotection 341-53 (Kamenka et al., eds. NPP Books) (1992). However, it has been surprisingly found by the present inventors that sigma receptors are prevalent on some types of cancer cells, e.g., neuroblastoma, melanoma, glioma, pheochromocytoma colon, renal, prostrate and lung carcinoma cells. Recently, John et al. have found that MCF-7 breast tumor cells express sigma receptors. John et al., J. Med. Chem. 37:1737-1739 (1994).
Sigma receptors exist in at least two distinct subtypes termed sigma-1 and sigma-2. S. B. Hellewell, et al., A sigma-like binding site in rat pheochromocytoma (PC12) cells: Decreased affinity for (+)-benzomorphans and lower molecular weight suggest a different sigma receptor form from that in guinea pig brain, Brain Res. 527:244-253 (1990); R. Quirion, et al., A proposal for the classification of sigma binding sites, Trends in Pharmacol. Science 13:85-86 (1992). Tritiated sigma ligand probes such as (+)-pentazocine (a sigma-1 selective ligand), W. D. Bowen, et al., .sup.3 H!(+)-Pentazocine: A potent and highly selective benzomorphan-based probe for sigma-1 receptors, Mol. Neuropharmacol. 3:117-126 (1993); B. R. de Costa, et al., Synthesis and evaluation of optically pure .sup.3 H!(+)-pentazocine, a highly potent and selective radioligand for sigma receptors, FEBS Letters 251:53-58 (1989); and tritiated I,3-o-ditolylguanidine (DTG, a sigma non-subtype selective ligand), E. Weber, 1,3-Di(2-5-.sup.3 H!tolyl)guanidine: a selective ligand that labels sigma type receptors for psychotomimetic opiates and antipsychotic drugs, Proc. Natl. Acad. Sci. USA 83:8784-8788 (1986), have been used to characterize the expression of sigma receptors on various human tumor cell lines and to establish the pharmacological profiles for various drugs.
A very high density of both sigma-1 and sigma-2 receptor subtypes have been expressed on many human and rodent tumor cell lines (Bmax=1000-4000 fmol/mg protein). B. J. Vilner, et al., Sigma-1 and Sigma-2 receptors are expressed in a wide variety of human and rodent tumor cell lines, Cancer Res. 55:408-413 (1995). High levels of sigma receptors have also been reported in membrane preparations obtained from surgically removed solid human tumor tissue using .sup.3 H!DTG. G. E. Thomas, Sigma and opioid receptors in human brain tumors, Life Sci. 46:1279-1286 (1990); W. T. Bem, et al., Overexpression of sigma receptors in nonneural human tumors; Cancer Res. 51:6558-6562 (1991).
Scatchard's analysis of 4-.sup.125 I!-4-(N-benzylpiperidin-4-yl)-4-iodobenzamide binding in human breast adenocarcinoma (MCF-7) cells revealed that breast cancer cells possess approximately a million receptors per cell. C. S. John, et al., Synthesis and pharmacological characterization of 4-125I!-N-(N-benzylpiperidin-4-yl)-4-iodobenzamide: a high affinity sigma receptor ligand for potential imaging of breast cancer, Cancer Res. 55:3022-3027 (1995).
A high density of sigma receptors has also been found on membranes prepared from human breast biopsy tissues. This can be compared to normal breast tissue, which has essentially no sigma receptors. C. S. John, et al., Characterization of sigma receptor binding sites in human biopsied solid breast tumors, J. Nucl. Med. 37:267P (1996)(abstract).
Furthermore, sigma receptors expressed in human melanoma cells, breast cancer cells, non-small cell lung carcinoma, and human prostate tumor cells have been characterized using different radio-iodinated sigma ligands. C. S. John, et al., A malignant melanoma imaging agent: synthesis, characterization, in vitro binding and biodistribution of iodine-125-(2-piperidinyiaminoethyl)4-iodobenzamide, J. Nuc. Med. 34:2169-2175 (1993); C. S. John, et al., Synthesis and characterization of .sup.125 I!-N-(N-benzylpiperidin-4-yl)-4-iodobenzamide, a new sigma receptor radiopharmaceutical: high affinity binding to MCF-7 breast tumor cells, J. Med. Chem. 37:1737-1739 (1994); C. S. John, et al., Sigma receptors are expressed in human non-small cell lung carcinoma, Life Sci. 56:2385-2392 (1995); C. S. John, et al., Characterization and targeting of sigma receptor binding sites in human prostate tumor cells, J. Nucl. Med. 37:205P (1996)(abstract); C. S. John, et al., Synthesis, in-vitro binding and pharmacokinetics of radioiodinated (N-benzylpiperidin-4-yl)-2-iodobenzamide: sigma receptor marker for human prostate tumors, (manuscript submitted).
From this information, the inventors concluded that sigma receptors were potential targets for the development of diagnostic probes. Consequently, the inventors embarked on a study of Tc-99m radiolabeled chelates that would bind to the sigma sites. Tc-99m is a widely used radionuclide in clinical nuclear medicine due to its instant availability from the Mo-99/Tc-99m generator, ideal physical properties (t.sub.1/2 =6.02 hrs; gamma energy=140 keV), absence of beta emissions, low radiation burden to patients and low cost.
However, it can be difficult to develop receptor based Tc-99m radiopharmaceuticals for diagnostic imaging. In order to provide in vivo stability to the radiolabel, Tc-99m has to be complexed to a chelate. Thus, it is necessary to synthesize a molecule suitable for imaging a particular receptor site such that it possesses a chelating moiety and a pharmacological moiety (i.e., a pharmacophore appended to the chelate moiety. For example, compounds specific for progestin receptors have been described. J. P. DiZio, et a., Technetium- and rhenium-labeled progestins: synthesis, receptor binding and in-vivo distribution of an 11-substituted progestin labeled with technetium-99 and rhenium-186, J. Nucl. Med. 33:558-569 (1992); J. P. O'Neil, et al., Progestin radiopharmaceuticals labeled with technetium and rhenium: synthesis, binding affinity, and in-vivo distribution of a new progestin N.sub.2 S.sub.2 -metal conjugate, Bioconjugate Chem. 5:182-193 (1994)
Furthermore, addition of a chelating moiety to a pharmacological entity often results in an increase in steric bulk of the molecule. Frequently, therefore, the molecule's affinity for the receptors can be compromised. John A. Katzenellenbogen, Designing Steroid Receptor-Based Radiotracers to Image Breast and Prostate Tumors, J. Nuclear Med. 36:8S-13S (1995).
There are other problems that arise when moving beyond imaging of tumors to treatment of tumors. Treatment of tumors, particularly the above mentioned tumors, can involve very large dosages of therapeutic materials. For example, chemotherapeutic materials may be administered in amounts on the order of 10-100 milligrams for a 70 kg normal adult. Such high dosages are needed because of the lack of specificity of the chemotherapeutic compounds for the tumors. The high dosages also lead to side effects, such as nausea, hair loss and vomiting, that can severely weaken a patient with a tumor or tumors. Therefore, it would be desirable to develop materials that could be administered in a much smaller amount, such that the severity of the side effects would be lessened.